Fix final round of gcc warnings relating to unused parameters.

* debug.cc (iscygthread): New function.
* debug.h: Declare it.
* exceptions.cc (set_process_mask): Flush pending signals.
(handle_sigsuspend): No need to flush pending signals.
(call_handler): Refine previous tests of muto ownership.  Only clear wait()'s
when we have definitely responded to a signal.
* fhandler_console.cc (fhandler_console::read): Don't set EINTR if executing in
a "cygwin" thread.
* sigproc.cc (proc_subproc): Use second argument to control whether CLEARWAIT
actually sets "signalled" flag.
* sync.h (muto): Add 'unstable' method.

winsup/cygwin/sync.h

@@ -37,6 +37,7 @@ public:
   /* Return true if caller thread owns the lock. */
   int ismine () {return tid == GetCurrentThreadId ();}
   DWORD owner () {return tid;}
+  int unstable () {return !tid && (sync || waiters >= 0);}
 };
 
 /* Use a statically allocated buffer as the storage for a muto */